pontoon n.

"bridge," in anatomy and in various Latin expressions, from Latin pons "bridge, connecting gallery, walkway," earlier probably "way, passage," from PIE root *pent- "to go, tread" (see find (v.)). Especially pons asinorum "bridge of asses," nickname since early 16c. for the fifth proposition of the first book of Euclid, which students and slow wits find difficulty in "getting over": if two sides of a triangle are equal, the angles opposite these sides also are equal. "The original allusion seems to have been to the difficulty of getting asses to cross a bridge" [Century Dictionary]. The Latin word is the source of Italian ponte, French pont, Spanish puente.

"flat-bottomed, square-ended, mastless river boat," c. 1500, perhaps a local survival of late Old English punt, which probably is from British Latin ponto "flat-bottomed boat" (see OED), a kind of Gallic transport (Caesar), also "floating bridge" (Gellius), from Latin pontem (nominative pons) "bridge" (from PIE root *pent- "to tread, go;" see find (v.)). Or from or influenced by Old French cognate pont "large, flat boat." Compare pontoon.